The present invention relates to a protective envelope for a semiconductor integrated circuit borne on a lead frame of metal.
More particularly, the invention relates to a protective envelope made of a plastics material for enclosing a semiconductor integrated circuit therein, which envelope comprises a flattened parallelepiped body having a sidewall formed of first and second portions set to converge toward each other, a lead frame embedded in said body and bearing the integrated circuit, the lead frame having a section bent to form a baffle plate orientated toward the first of said sidewall portions.
This envelope is formed by injecting a resin in a molten state into a mold accommodating the lead frame of conductive material with the integrated circuit on it. The mold, usually built in two halves, is opened after curing the resin to release the finished package.
The invention further relates to a mold for use in molding said envelope.
In the field of semiconductor (usually silicon) integrated circuits borne on a metal lead frame, there exists a growing demand for such protective envelopes to be of little bulk and capable of keeping the integrated circuit fully isolated electrically in a way that would not interfere with the heat transfer capabilities required of the envelope in order to dissipate heat generated on its interior during the circuit operation.
A silicon wafer formed with the integrated circuit is assembled with a lead frame made of a conductive material. This lead frame is operative to communicate the electric signals from the circuit to the outside world on a plurality of contact pins that are connected to respective pads on the integrated circuit, as well as to serve the heat transfer function demanded of it. The lead frame additionally serves a structural function by supporting the wafer not only during its operation, as mentioned above, but also in the course of its fabrication, as explained hereinafter.
For the circuit to operate as expected, the wafer must be kept suitably isolated from any external influence likely to alter the features of the electric signals.
For the purpose, the wafer is confined, together with the lead frame in contact with it, inside an envelope of an insulating plastics material, such as a suitable resin, which will keep all its parts isolated from the outside world but for its portion in contact with the lead frame, this portion being to dissipate heat as mentioned.
The lead frame is in the form of a conductive material foil, no less wide than the wafer, having a first surface to bear the wafer, and a second surface, the heat transfer surface, on the opposed side from the first.
Heat transfer takes place through the second surface, but this surface requires to be isolated electrically and, accordingly, is enclosed in the envelope. The insulator should not be thick enough to impair the transfer of heat, yet it is expected to provide an adequate degree of electrical isolation.
The integrated circuit industry is concerned with Microsystems, i.e., systems of a minute physical size, and this factor is at the root of the great difficulties encountered when a heat transfer surface is to be provided in still smaller size than the circuit itself.
Briefly, the heat transfer surface should be covered with a layer of insulating material that is both thin and uniform. This layer is formed from a suitable resin, the resin being injected in a molten state into a mold that accommodates the silicon wafer and part of the lead frame. The mold is opened after curing the resin.
A commonly occurring problem is that the molten resin tends to flow unevenly through the mold interior by reason of a high viscous drag opposing the penetration with resin of the shallow heat transfer gap provided between the second surface and the bottom half of the mold. This situation of uneven flow leaves regions of the heat transfer surface uncovered with resin, resulting in inferior electrical or electronic protection of the surface.
The problem has been addressed in the state of the art by Japanese Patent No. JP1268159 to NEC Corp., wherein the conductive frame is formed with a bent section jutting toward the injection hole of the mold. In this way, the inflow of molten resin undergoes a compressive action effective to overcome its viscous drag and force it to the heat transfer area. A denser and adequately thin layer of resin is thus obtained.
In this reference, the end portion of the holder inside the mold is bent through an angle of 35xc2x0 to 40xc2x0 from the remainder of the holder to reach a position just opposite to the injection hole of the mold.
The results obtained with this arrangement do point to this orientation providing a useful baffle plate, though not one that can yield an unfailingly uniform thin layer of resin under all the critical flow conditions met by the injected stream of resin.
Other approaches to providing integrated circuits with envelopes incorporating a thin heat transfer layer, also involve constricting the mold injection conduit in various ways, e.g., as disclosed in U.S. Pat. No. 5,793,613 and U.S. Pat. No. 5,935,502. These references, although achieving improved compression of the resin stream, are still far from ensuring a uniform distribution of the resin.
Thus, the state of the art offers no effective methods to solve the technical problem just described.
An embodiment of the present invention provides an envelope for integrated circuits with structural and functional features appropriate to electrically isolate a heat transfer surface coated with a thin resin layer and allow a transfer of heat to occur as required for proper operation of the integrated circuit.
A principle on which the present invention stands is one of using a special mold, accommodating a silicon wafer borne on a metal lead frame, to produce a fully insulative envelope that also provides for an adequate transfer of heat. This result can be achieved by facilitating the introduction of a molten resin stream into the gap between the underside of the lead frame and the inner surface of the bottom mold half, by application of a sufficient pressure to overcome the viscous drag arising from the shallowness of the gap and the flow characteristics of the resin in the molten state.
In one embodiment, the bent section of the lead frame ends in a plane edge extending parallel to said first sidewall portion at a spacing therefrom.
The features and advantages of a protective envelope according to the invention will be apparent from the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings.